An image forming method by electrographic process generally comprises: a charging process of charging by electric discharge the surface of a photoconductor which is an image bearing member; an exposing process of forming a latent electrostatic image by exposing the charged photoconductor surface; a developing process of developing a toner image by supplying a toner to the latent electrostatic image formed on the photoconductor surface; a transferring process of transferring the toner image on the photoconductor surface onto the surface of a transferring member; a fixing process of fixing the toner image on the surface of the transferring member; and a cleaning process of removing the toner left on the surface of the image bearing member after the transferring process.
Recently, in association with the broad propagation of color image forming apparatuses using the electrographic process and easily available digitized images, it is desired to make an image to be printed with higher definition. While higher resolution and gradation of images are being examined, regarding the improvement of a toner which visualizes a latent image, it has been studied to further ensphere the toner and reduce the particle size in order to form an image with high definition. Since the toners produced by pulverizing methods have limited properties thereof, polymerized toners produced by a suspension polymerization method, an emulsification polymerization method and a dispersion polymerization method capable of ensphering the particles and reducing the particle size are being employed.
A toner with high sphericity is easily affected by electric lines of force in an electrostatic development method, and a toner image is closely developed along the electric lines of force of the latent electrostatic image on the photoconductor. The toner is easily arranged densely and uniformly, and the reproducibility a thin line becomes high when a latent image of fine dots is reproduced. In addition, since the toner has high particulate flowability due to its smooth surface and small adhesive strength among the toner particles or between the toner particles and the photoconductor; Therefore, the electrostatic transferring method has high transfer ability since the toner is easily affected by the electric lines of force, and the transfer is easily performed faithfully along the electric lines of force.
However, the toner with high degree of sphericity has a smaller surface area compared to an amorphous toner of the same particle diameter. This means that the surface available for frictional charge by contact with a frictional charging member such as a magnetic carrier and developer regulation member is small. When the toner is spherical, it easily slides on the surface of the above frictional charging member. Thus, the charging speed and the charging level are low. Therefore, more than a certain amount of a charge controller is required on the surface of the toner.
Additionally, the smaller the toner particle diameter is for enhancing the reproducibility of a fine dot, the more the frictional charge property is reduced. Thus, it is a critical issue to simultaneously satisfy the charge property, developing property and transfer property.
Regarding the spherical toner and the toner having a small particle diameter, various proposals to control their shapes have been made. Shape Factors SF-1 and SF-2 are often used as indicators representing the toner shape. The shape factor SF-1 is an indicator that represents the degree of roundness of a toner particle, and SF-2 is an indicator that represents the degree of convexoconcave of a toner particle. For example, Patent Literatures 1 to 3 attempt to control the shape of the particles by defining the ranges of the shape factors SF-1 and/or SF-2 in order to simultaneously satisfy the charge property, developing property, transfer property, or cleaning property even with a spherical toner and a toner having small particle diameter.
Patent Literature 4 describes a technology which defines the range of the shape factor of toner particles as well as a surface area ratio represented by the following formula:Surface area ratio=ρ×D50p×S where ρ is a specific gravity (g/cm3) of the toner particles, D50p is a number average particle diameter (m) of the toner particles, and S is BET specific surface area (m2/g) of the toner particles.
This surface area ratio is a different measure that represents the degree of convexoconcave of the toner particle from the foregoing shape factor. When the value of the surface area ratio exceeds a given range, the degree of convexoconcave on the surface of the toner particle becomes large. This allows an external additive added externally with time to the toner particles to enter in the depressing portions of the toner particles, and thus it becomes impossible to maintain the charge property and the transfer property over a long period of time.
In Patent Literature 5, the toner surface is defined by an atomic force microscope. However, the degree of convexoconcave in Patent Literature 5, i.e., the degree of surface roughness (Ra), the degree of standard deviation of Ra (RSM) and the number of projective portions having difference of elevation of 20 nm or more, is not sufficient for a cleaning system with higher durability and stability. It was desired that more enhanced cleaning property, by further increasing the degree of convexoconcave in the toner shape reduced cleaning blade abrasion and hence improved the image stability in printing with time.
The technology described in Patent Literature 5 is characterized by having a fine convexoconcave on the toner surface for the purpose of particularly improving the charge property, developing property and transfer property. However, the irregular shape property (large convexoconcave) of the toner as a whole is not sufficient since fine particles such as organosilica are not suitably used for changing the shape. Therefore, it was absolutely insufficient for solving a problem of an stability of the cleaning over time.
As described above, despite many attempts to enhance the charge property, developing property, transfer property or cleaning property by controlling the shapes of the toner particles, a toner having sufficiently satisfactory performance and the related technology have not yet been provided because all the attempts only have a rough view of the surface shape of the toner particle, and none of them grasps the microscopic state of concavoconvex for simultaneously satisfying the charge property, developing property, transfer property and cleaning stability with time.
Patent Literature 1Japanese Patent Application Laid-Open(JP-A) No. H09-179331Patent Literature 2JP-A No. H10-142835Patent Literature 3JP-A No. H11-327197Patent Literature 4JP-A No. 2001-51444Patent Literature 5JP-A No. 2004-246344